What is the role of a lexer in a compiler?

A lexer in a compiler is responsible for breaking down the source code into meaningful chunks, known as tokens.

In the process of compiling a program, the lexer, also known as the lexical analyser, plays a crucial role. It is the first phase of a compiler. Its main job is to read the input characters and produce as output a sequence of tokens that the parser uses for syntax analysis. Each token represents a logically cohesive sequence of characters, such as an identifier, keyword, or operator.

The lexer works by scanning the source code from left to right, character by character. It categorises each character and groups them into tokens based on the rules of the programming language. For example, in the C programming language, the string 'int' would be recognised as a keyword, while 'x' might be recognised as an identifier.

The lexer also removes from the source code any white space and comments, which are not needed for the actual compilation process. This is known as 'lexical noise'. By removing this noise, the lexer simplifies the job of the subsequent stages of the compiler.

In addition to breaking down the source code into tokens, the lexer also checks for lexical errors. These are mistakes in the source code that violate the rules of the programming language. For example, in many languages, identifiers (names of variables, functions, etc.) must start with a letter. If an identifier starts with a number, the lexer would flag this as a lexical error.

In summary, the lexer is a vital component of a compiler. It performs the initial processing of the source code, breaking it down into tokens, removing lexical noise, and checking for lexical errors. This prepares the source code for the next stage of the compiler, the parser, which checks the syntax of the program.